Add json_document and json_view: node index, parser, and document

Add json_document and json_view, a read-only, zero-copy index of a
JSON text, as the first public slice of the zero-copy view (#5295).

A parse produces a flat array of 16-byte nodes in document order,
one per value and one per object key. Strings stay in the source
text; escaped strings are decoded into an arena. Integers are
converted while their digits are in the cache; floats keep only
their digit layout and are converted on read. Containers store the
size of their subtree, so a reader can step over one in constant
time. A document makes a handful of allocations, however many
values it has.

The parser accepts exactly what json::parse accepts, with every
combination of ignore_comments and ignore_trailing_commas, with and
without a trailing NUL, and under JSON_STRICT_NUL_HANDLING. It is
portable C++11 and does not depend on byte order.

basic_json_document adds parse, parse_copy, accept, read (reuses a
document's memory), root, is_discarded, source, owns_source,
node_count, memory_usage, and shrink_to_fit. basic_json_view adds
type, the is_* queries, operator bool, size, empty, materialize,
and source_offset. A parse error throws the same exception
basic_json::parse would throw for the same input, message and
position included.

detail::abi_config keeps JSON_STRICT_NUL_HANDLING and
JSON_USE_LEGACY_DISCARDED_VALUE_COMPARISON readable after json.hpp
undefines them, in the ABI namespace so they always match the
basic_json in use.

A NUL byte that ends a // comment is the end of the input, as in
parse() since #5696.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

docs/mkdocs/docs/examples/basic_json_document__read.cpp

@@ -0,0 +1,31 @@
+#include <iostream>
+#include <string>
+#include <vector>
+#include <nlohmann/json_view.hpp>
+
+using json_document = nlohmann::json_document;
+
+int main()
+{
+    std::cout << std::boolalpha;
+
+    std::vector<std::string> messages =
+    {
+        R"({"id": 1})", R"({"id": 2, "tag": "x"})", R"({"id": 3})"
+    };
+
+    // parse into the same document over and over: its node index and decode
+    // buffer are reused instead of being freed and reallocated for each message
+    json_document doc;
+    std::size_t total = 0;
+    for (const auto& msg : messages)
+    {
+        doc.read(msg);
+        total += doc.root().size();
+    }
+    std::cout << total << '\n';
+
+    // read() can also change what kind of input is owned/borrowed between calls
+    doc.read(std::string(R"({"owned": true})"));
+    std::cout << doc.owns_source() << '\n';
+}